1

CROP PROTECTION PROGRAMME

Identification of resistance to major nematode pests of yams (Dioscorea

spp.) in West Africa

R6694 (ZA0021)

FINAL TECHNICAL REPORT

1 APRIL 1996 – 31 MARCH 2000

RICHARD PLOWRIGHT & CHARLES KWOSEH

CABI Bioscience

Bakeham Lane

Egham Surrey TW20 9TY.

"This publication is an output from a research project funded by the United Kingdom Department for International Development for the benefit of developing countries. The views expressed are not necessarily those of DFID.” [R6694 Crop Protection Programme]

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EXECUTIVE SUMMARY

This DFID, CPP project was a collaboration between CABI Bioscience and the Tuber and

Root improvement programme of the International Institute of Tropical Agriculture in

Nigeria. The project sought to identify sources of resistance to the principal nematode

pests of yams (Scutellonema bradys and Meloidogyne incognita) which cause dry rot and

galling of yam tubers in West Africa and are a priority for yam breeders at IITA. The

project focused on development of yam nematode resistance screening techniques for field

and screenhouse and carried out extensive screening of germplasm from West Africa. An

important aim was to achieve a better understanding of the genetic and environmental

components of variability in the host: parasite interaction of yams and yam nematodes

through strategic research. The project involved survey work, which examined the

distribution of nematodes in Ghana and assessed farmer’s perceptions of nematode losses.

It also involved field, screenhouse and glasshouse trials in Nigeria, Ghana and UK.

Yam nematodes are widespread in Ghana, where they are well known to farmers and

where attempts to control nematodes using fallow and selecting good planting material are

commonplace. Farmers in Ghana consider losses from S. bradys to be more important

than those from M. incognita. Farmers grow a large number of varieties and whilst they

might be willing to test new, potentially resistant, varieties, it is important that such should

fit into this diverse background.

Experiments on precision established that Latin square field designs are more favourable

than an Alpha array, but neither dealt satisfactorily with the heterogeneity of natural

nematode populations and thus screening protocols which used artificial inoculum

(nematode infected yam peelings) were more reliable. Five or more replications are

recommended in order to approach meaningful separation of quantitative variants. Data

from experiments using young yam seedlings should be interpreted with care since this

work identified discrepancies between the results of such experiments and expression in

the field.

Most of the yam species and accessions screened have been found to be susceptible to

nematodes. There was variation in susceptibility within Dioscorea rotundata, but the

functional importance of this variation in terms of increased yield and reduced storage

losses requires more work. Dioscorea dumetorum has resistance to both S. bradys and M.

incognita, but barriers to interspecific crosses to D. rotundata preclude the exploitation of

this resistance at present.

Monoxenic culture techniques for the mass rearing of S. bradys have successfully been

developed, which can assist further detailed pot experimentation.

Pratylenchus coffeae which is widespread in Ghana on Musa spp. can multiply in roots of

a number of species of Dioscorea, namely D. alata, D. rotundata, D. cayenensis, D.

esculenta, D. dumetorum, D. bulbifera, but does not multiply readily in yam tubers and

hence is not a threat to yam production. P. coffeae in Ghana appears to be a Musa host

race.

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BACKGROUND

Yams, Dioscorea spp. are important economic crops grown mainly in tropical countries

especially, in the yam belt or yam zone of West Africa. Many farm-families depend on the

tubers for food, cash, local food security and other traditional uses. The tubers provide a

substantial intake of vitamins (thiamine and vitamin C) and iron and also protein. Yam

peels and over matured tubers are used to feed domestic animals such as pigs, goats and

chickens. It is gradually being realised as an important non-traditional crop for export.

However, yams are severely damaged by plant parasitic nematodes reducing yield, food

quality, and market value. At least three potential nematode pest species namely

Scutellonema bradys, Meloidogyne spp. and Pratylenchus coffeae are known to exist.

Reduction of 20-30% in tuber weight at harvest has been reported. Also, nematode

infection contributes to long term storage losses, which have been estimated as 50%, and

loss can be total.

Chemicals, cultural methods such as hot water treatment of seed yams, crop rotation,

biological control, the use of resistant varieties and integrated management, can control

plant-parasitic nematodes. Most of the management options are limited in use due to high

costs, time, feasibility and adverse effects on the environment and mammalian toxicity.

Most plant-parasitic nematodes by their nature and habitat are restricted in their movement

from field to field and area to area and new physiological races that break resistance

probably would be slow. Yam cultivars with resistance to nematodes would provide an

attractive nematode management option appropriate to smallholder farm families. There is

therefore the need to begin to search for nematode resistance in yams.

PROJECT PURPOSE

The main project purpose was to identify sources of resistance to yam nematodes for use in

Yam Improvement Programmes aimed at reducing the yield and storage losses in yam

caused by nematodes. This addressed the DFID Crop Protection Programme’s

Forest/Agriculture Interface, Purpose 1, output of improved methods of managing the

major nematode pests of root and tuber crops. The project aimed to achieve a better

understanding of the genetic and environmental

components of variability in the host: parasite interaction of yams and yam nematodes

through strategic research. The project focused on development of yam nematode

resistance screening techniques and screening of yam germplasm for nematode resistance

then examined the correlation between nematode host status and other plant characteristics.

The findings should provide recommendations for plant breeders.

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DETAILS OF COMPLETED RESEARCH ACTIVITIES

OUTPUT 1. TECHNIQUES FOR SCREENING YAMS FOR RESISTANCE TO

NEMATODES REFINED

MONOXENIC CULTURE OF THE YAM NEMATODE, S. BRADYS

Yam nematodes have not previously been cultured under monoxenic conditions, but with

careful maintenance, monoxenic cultures of migratory nematodes can provide a reliable

supply of inoculum of consistent quantity and quality as are required for screening. In this

work, the suitability of excised tuber tissues as a tissue substrate for S. bradys was

investigated. White yam tuber was washed and cut into pieces with skin intact and then

dipped in Bio- Supercarb for 15 min. The pieces were air dry and then cut into slices of

about 3-6 g each with skin intact. The skin of each slice was removed and then sterilised in

sodium hypochlorite (1 % available chlorine) for 15 min. The slices were rinsed six times

with sterilised distilled water and then dabbed dry on filter paper. The sterilised yam slices

were plated on 1 % water agar medium in Petri dishes. The plates had been put in a drying

cabinet at 30 C for 10 min to dry off excess water. The plates were kept in the culture room

at 25 C for about three weeks to callus before inoculated with S. bradys. S. bradys was

sterilised with 5 drops of 0.1 % malachite green (technical grade) for 5 min and then rinsed

10 times with sterilised distilled water. Each plate containing the callus yam slice was

inoculated with 20 or 30 active juvenile and adult stages of S. bradys. The plates were

sealed with insulating tape and kept in the culture room at 25 C.S. bradys were then

extracted from both the yam tissue and agar of the plates by a modified Baermann funnel

method after five months.

A means to suppress contaminating fungi in yam tissue callus was needed and Bio-

Supercarb, a systematic fungicide, which contains carbendazin, was used. The mortality of

the fungicide was tested on S. bradys. Three levels of concentration of the fungicide, 0.5,

1.1, and 2.2 and litres of water were used and water as the control. About 5 ml of the

various fungicide concentrations and water were separately dispensed into sterilised watch

glasses. Twenty active stages of S. bradys were put in each watch glass; there were three

replicates. S. bradys was obtained from yam bin cultures at International Institute of

Parasitology (IIP), UK. The nematodes were observed daily for three days and the effect of

treatment on them assessed by recording the number of live and dead nematodes. A

nematode was considered alive when active.

REFINEMENT OF A PROTOCOL FOR SCORING SYMPTOMS OF NEMATODE INJURY AND

QUANTIFYING NEMATODE DENSITIES

Experiments were done to examine the rate of emergence of S. bradys from chopped yam

tissues into water using a Baermann funnel, in order to establish optimum period of

incubation with regard to nematode recovery and the deterioration of yam tissue. A novel

method for scoring nematode symptoms on yam tubers was also devised and tested.

Although not critically evaluated experimentally, different methods were operated

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concurrently in the same laboratory at IITA allowing full discussion of the practical

elements of each.

The novel method began with a random selection of tubers from each accession.

Symptoms of dry rot, galling as well as tissue softness and cracks were then recorded for

each tuber (0-3) based on the Table 1.1.

Table 1.1 Description of infection types and scores for yam nematodes

Infection Softened yam

Tuber tissue (%)

Root/tuber

galling

Tuber

Cracking

Root

Necrosis (%) score Dry/wet rot

0 0 None None None None

1 < 25 Light Light Light Light

2 26 – 50 Moderate Moderate Moderate Moderate

3 > 50 Severe Severe Severe Severe

Based on the scoring, the selected tubers of each accession were grouped according to

symptom types. A sub-sample of one to four tubers for each accession was selected from

the groupings. Each of the selected tubers was separately peeled (strip) from the proximal

to the distal end at three places of about equal distance from one another. The peelings of

each tuber were bulked, chopped to about 3-4 mm width and 2 cm length and thoroughly

hand-mixed. A 5 g sub-sample was taken for nematode extraction by a modified Baermann

funnel method. The nematodes were identified and numbers were counted. Sub-samples of

3 g in muslin bags were taken to assess the population density of Meloidogyne species. The

tissues in muslin bags were boiled in acid fuchsin stain for 15 min. and each muslin bag

was macerated for 20s at low speed. Stained Meloidogyne species in juvenile stages and

adults were counted.

COMPARISON OF FIELD EXPERIMENTAL DESIGNS

Two field experiments were set up at IITA research farm, Ibadan, to compare the

efficiency of Latin square arrangement and Alpha array experimental designs for screening

for yam nematode resistance. The experiments were established with alternating

allocations of naturally infested soil and naturally infested soil plus artificial infestation

with S. bradys infected yam tuber peelings. The naturally infested field had been cropped

to yam for three consecutive seasons.

STUDIES ON THE INVASION AND POPULATION DYNAMICS OF S.BRADYS IN YAM TUBERS.

A study of the population dynamics of yam nematode in tubers was done to examine the

role of both the rate of nematode infection and multiplication on observed differences in

susceptibility. This field trial studied the invasion and population dynamics of S. bradys in

resistant and susceptible yam tubers at IITA.

INVESTIGATION OF THE INOCULUM TYPE AND SIZE OF S. BRADYS

A range of nematode delivery systems are available for inoculating nematodes. This work

was done to optimise the delivery and quantity of inoculated nematodes in yam nematode

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screening protocols. Two types and three sizes of inoculum were used in this study in the

screenhouse at IITA. The types of inoculum are nematode in water suspension and

chopped yam peelings. The inoculum sizes were 1 g, 25 g and 100 g of chopped S. bradys

infected yam peelings and 120, 2,800 and 10,000 S. bradys active stages representing the

number of nematodes in the peelings respectively. Two contrasting IITA D. alata

accessions, TDa 294 and TDa 92-2 were used. The plants were potted in about 800 ml

heat-sterilised 2:1 topsoil-cocopeat mix and then were inoculated after two weeks. A

simple line screening design with five replicates was used. The plants were assessed 9

weeks after inoculation by determining any visual nematode damage and reproduction in

roots and tubers of all entries in the screen.

INFLUENCE OF REPLICATION ON PRECISION IN ESTIMATING QUANTITATIVE DIFFERENCES

IN SUSCEPTIBILITY OF YAM TO NEMATODES

There are currently no good sources of resistance in Disoscorea rotundata to yam

nematode, although work, to date, suggests that quantitative variation might play a role.

This work was done to examine the level of replication that might be required to enable

selection of quantitative variants. A set of 21 potted tissue culture plants of two contrasting

D. rotundata accessions, TDr 87/00571 and TDr 87/00211 were used in this trial in the

screenhouse at IITA. The plants were potted in steam sterilised soil-cocopeat mix. The

established plants were inoculated with about 1,700 active stages of S. bradys (35 g

infected yam peelings) per pot. The simple line design was used. Entries were assessed

eight weeks after inoculation by recording any visual nematode injury and the number of S.

bradys in roots or tubers. The plants were randomly harvested as groups of 3, 5 and 13

representing the number of replicates for each entry.

OUTPUT 2 YAM GERMPLASM SCREENED FOR RESISTANCE TO

NEMATODES

PRELIMINARY SCREENING FOR NEMATODE RESISTANCE OF IITA YAM GERMPLASM

At the outset of this research project an assessment of nematode resistance in stored yam

tubers at IITA was made. This was a collection of germplasm which had not been exposed

experimentally to nematode infection. For this reason any potentially resistant tubers

identified, had to be treated with caution. A total of 306 D. rotundata and 18 D. alata

accessions were screened at International Institute of Tropical Agriculture (IITA), Ibadan,

Nigeria. The tubers were harvested in 1996 from the research farm at IITA headquarters,

Ibadan, Nigeria. The plots were planted with cassava or yam in 1994 and mucuna fallow in

1995. The harvested accessions were either stored in a storeroom maintained at 19 C or

kept in an open-air yam barn under ambient conditions for about three months. Each

accession was randomly selected and symptoms of dry rot, galling and tissue softness as

well as cracking of each tuber were scored (0-3). Based on the scoring, the selected tubers

of each accession were grouped according to symptom types. A sub-sample of one to four

tubers for each accession was selected from the groupings. Each of the selected tubers was

separately peeled (strip) from the proximal to the distal end at three places of about equal

distance from one another. The peelings of each tuber were bulked, chopped to about 3-4

mm width and 2 cm length and thoroughly mixed. A 5g sub-sample was taken for

nematode extraction by a modified Baermann funnel method. The nematodes were

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identified and numbers were counted. Sub-samples of 3 g in muslin bags were taken to

assess the population density of Meloidogyne species. The tissues in muslin bags were

boiled in acid fuchsin stain for 15 min. and each muslin bag was macerated for 20 s at low

speed. Stained Meloidogyne species in juvenile stages and adults were counted.

SCREENHOUSE SCREENING OF SELECTED GENOTYPES OF IITA YAM ACCESSIONS,

NIGERIA.

Based on the preliminary assessment of nematode infestation in yams of the IITA yam

breeding programme, 23 genotypes of D. rotundata and 17 wild and domesticated

Dioscorea species were selected for screening in pots. The wild and some domesticated

yams were collected from Cameroon, Benin Republic, Republic of Guinea, Cote d'Ivoire,

and Burkina Faso all in West Africa by the Biotechnology Research Unit of IITA. Uniform

plants from yam minisetts pre-sprouted in sterilised moist coco-peat (shredded coconut

husk) were used for screening for resistance to S. bradys and Meloidogyne spp. A

randomised complete block design with eight replicates was used. Established plants in the

pots were inoculated with about 360 active stages of S. bradys per pot and 500 juveniles of

Meloidogyne species per pot for each experiment. All entries were assessed five months

after inoculation by recording visual nematode damage and the number of nematodes in

roots or tubers.

FIELD SCREENING OF GHANAIAN LANDRACES FOR RESISTANCE TO S. BRADYS AND

MELOIDOGYNE SPECIES

Forty Ghanaian yam landraces identified by farmers during the Farmer participatory

appraisal (See output 3) were screened for resistance at Crops Research Institute in

Kumasi.

GLASSHOUSE INVESTIGATION OF THE VARIATION IN SUSCEPTIBILITY OF YAM SPECIES TO

S. BRADYS, UK.

This experiment was set up at IIP, UK in the glasshouse. The yam plants were obtained

from yam vines except D. dumetorum that was from seeds. They were screened in vacapots

containing about 200 ml sterilised soil-sand mix. The established plants were inoculated

with about 500 active stages of S. bradys. All the plants in the screen were assessed 9

weeks after inoculation by recording any apparent nematode damage and reproduction of

the nematodes in the roots or tubers.

FIELD SCREENING OF OPEN-POLLINATED YAM SEEDLING POPULATION FOR S. BRADYS

RESISTANCE.

The open-pollinated yam seedlings were obtained from accession TDr 95/05576 (female)

and suspected male, D. rotundata var. Puna. The seeds were sown in moistened peat

pellets in the glasshouse at IITA. The seedlings were transplanted to the field at 2 to 3 leaf

stage (5–6 weeks old) in ridges at 1 m x 0.25 m and then a shed was raised over them. 16-

16-16 NPK fertiliser was applied using the ring method. The seedlings were staked

immediately they started to crawl. Six weeks after transplanting, the seedlings were

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inoculated with about 1,700 S. bradys (25 g yam peelings). All the plants in the screen

were assessed seven months after inoculation by recording any apparent nematode damage

and reproduction of the nematodes in tubers. Each seedling was considered genotypically

different and there was no replication.

SCREENHOUSE SCREENING OF YAM PARENTAL LINES FOR RESISTANCE TO S. BRADYS

Yam parental lines of the Cellular and Molecular Technologies Laboratories, IITA were

tested in a pot experiment in the screenhouse for their reaction to S. bradys. Tissue culture

plants of ten parental lines (five each of D. alata and D. rotundata) were screened. The

tissue culture plants were potted in steam sterilised soil-cocopeat mix. The established

plants were inoculated with about1,700 active stages of S. bradys (35 g chopped S. bradys

infected yam peelings) per pot.

SCREENHOUSE SCREENING OF PARENTS AND PROGENIES OF INTRASPECIFIC

HYBRIDISATIONS FOR RESISTANCE TO NEMATODES

Intraspecific hybrids of TDr 87/00211 (male) and TDr 87/00571 (female) were screened in

pot experiments in the screenhouse for their reaction to S. bradys and Meloidogyne

incognita at IITA, Nigeria. Fifty-nine genotypes of intraspecific hybrids (Tissue culture

plants) and their parents were screened for their reaction to S. bradys and twenty-five for

Meloidogyne incognita. The tissue culture plants were potted in sterilised soil-cocopeat

mix. The established plants were inoculated with about 780 active stages of S. bradys (25 g

S. bradys infected yam peelings) per pot. For the Meloidogyne species trial, each pot was

inoculated with 400 juveniles of Meloidogyne species. The plants were assessed eight

weeks after inoculation by recording visual nematode injury and the number of nematodes

in roots or tubers.

OUTPUT 3 NEMATODE VARIABILITY ON YAM KNOWN

PARTICIPATORY RURAL APPRAISAL: NEMATODE PESTS PROBLEMS OF YAM IN GHANA

A farmer participatory appraisal of pests and diseases in stored yams was made in Ghana

in 1998 to examine farmers’ perceptions of nematode disease problems on yams. A total of

17 districts and farmers from 32 towns and villages were considered. The surveyed area is

within the Forest-transitional and Guinea-Savannah agroecological zones. The farmers

provided information on the type of varieties they grow, cropping system, planting

material, storage methods, and pests’ problems of yams and control. A total of 243

respondents (farmers) contributed. Dry rot and galling of tubers were identified. Nematode

infected yam tubers from all the sites visited were collected. Local and traditional yam

varieties were also collected for controlled nematode resistance screening.

IDENTIFICATION AND DISTRIBUTION OF YAM NEMATODES IN GHANA

Nematode populations in yam tissues collected during the rural appraisal were extracted

and identified to species level.

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HOST RACE STATUS OF PRATYLENCHUS COFFEAE ON YAM IN GHANA

Pratylenchus coffeae is widespread in Ghana, where it is a pest of Musa, but it was not

found infecting yam although, it is known to be a pest of yam in Pacific rim countries and

central America. This experiment was done to examine race specific variation in P. coffeae

in Ghana.

This trial was set up in the screenhouse at UST, Kumasi. Plants from yam minisetts and

tissue culture plants of Musa species were potted in sterilised soil-cocopeat mix. The

established plants were inoculated with about 800 active stages of P. coffeae (45 g infected

roots and corms of Musa). The plants were assessed eight weeks after inoculation by

recording visual nematode injury and the number of nematodes in roots or tubers.

Concurrently glasshouse experiments were done in the UK comparing the multiplication of

populations of P. coffeae from Belize and Ghana on Musa cv Grande Naine, Dioscorea

rotundata and D. togoensis.

RESEARCH RESULTS

OUTPUT 1. TECHNIQUES FOR SCREENING YAMS FOR RESISTANCE TO

NEMATODES REFINED

MONOXENIC CULTURE OF THE YAM NEMATODE, S. BRADYS

S. bradys populations in the inoculated yam tuber slices after treatment increased by ca

100 to 247 and 340 to 820 fold for 20 or 30 S. bradys after five months respectively (Table

1.2). Mean reproduction increased by 196 fold for 20 S. bradys and 600 fold for 30 S.

bradys inoculated (Table 1.2). The outer surfaces of the yam slices were darkened. This

may be due to necrosis of invaded tissues, whereas the inner parts had fresh tissues. All the

culture plates were free from contaminating microorganisms. Dry rot symptoms

characteristic of S. bradys infection in yam tissues were not observed. This is probably

because of the humid condition in the Petri plates. Also, nematodes are known to behave

differently in callus plant tissues. In previous work at IITA culturing of Scutellonema

bradys in the greenhouse was successful on sweet potato and cowpea, but not on sprouting

yam tuber pieces. However, this technique using sterilised yam slices on water agar

medium in Petri dishes can support high reproduction of S. bradys and constitutes a

suitable method for mass-production of the nematodes for screening for host plant

resistance. It is impossible to track the fate of inoculated nematodes, but it is always the

case that a proportion of nematodes are not potent. These results suggest that raising the

inoculum level from 20–30 dramatically improved the multiplication of nematodes in the

time frame of the experiment.

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Table 1.2. S. bradys reproduction in yam tuber slices five months after inoculation

Plate no.

Weight of yam

Slices/ g

Initial S. bradys

Population (Pi)

Mean no.

S. bradys (Pf)

Pf/Pi

1 3.0 20.0 2040.0 102.0

2 5.0 20.0 5230.0 261.5

3 3.0 20.0 3454.0 172.7

4 4.0 20.0 4930.0 246.5

5 6.0 30.0 24600.0 820.0

6 6.0 30.0 22800.0 760.0

7 3.0 30.0 10250.0 341.7

8 4.0 30.0 14400.0 480.0

Table 1.3. Effect of Bio-Supercarb concentration on S. bradys with initial inoculation

of 20 nematodes

Dosage

*Mean no. S.

bradys alive

Mean no. S.

bradys dead

% Live

nematodes

% Dead

Nematodes

Recommended (1.1) 19.0 1.0 95.0 5.0

Low (0.5) 20.0 0.0 100.0 0.0

High (2.2) 20.0 0.0 100.0 0.0

Tap water (control) 20.0 0.0 100.0 0.0

*Mean of three replicates

No death was observed in the low, high and control treatments after three days however,

one death was observed in the recommended dosage (Table 1.3). Analysis of variance

showed that there were no significant differences between treatments. It is very difficult to

axenise yam tissues for monoxenic culture, but Bio-Supercarb can be used to treat yam

slices for the suppression of fungi growth during the monoxenic culture of S. bradys.

REFINEMENT OF A PROTOCOL FOR SCORING SYMPTOMS OF NEMATODE INJURY AND

QUANTIFYING NEMATODE DENSITIES

The 0-3 rating of dry rot symptoms (Table 1.1) worked well as a means of assessment and

it was not thought that the creation of further divisions of symptom severity would lead to

increased accuracy. Peeling yam tissue from the proximal to the distal end reduces bias in

selecting areas of tissue for nematode population estimates, but smaller samples of peel

taken systematically over the tuber would probably work just as well.

Results from Baermann funnel extracts in water showed that most nematodes escaped from

macerated yam peelings, within two days of setting up the incubation at ambient

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temperatures. Incubated tissues rapidly deteriorated after 2 days, but the extra work

required for daily removal and replenishment of extracts, which does increase their

longevity, did not recover significantly more nematodes and did not justify the extra work

involved. This method would in any case be impossible within very large screening trials.

COMPARISON OF FIELD EXPERIMENTAL DESIGNS

The dry rot scores and nematode counts for the tested accessions are given in Tables 1.4

&1.5. The consistency and reliability of field screening was greatly improved by artificial

inoculation of nematodes and such methods, i.e. which do not rely on natural infection, are

recommended. Neither design eliminated the occurrence of false negative (resistant)

reactions as a result of ’escapes’ from natural nematode infection. Such escapes can occur

because of the heterogeneous distribution of nematodes in the soil. Generally, the Latin

square arrangement recorded more consistent dry rot rating and high nematode counts than

the Alpha array and gave small (4%) improvements in efficiency over the Alpha array.

Latin square which also benefits from being relatively simple to use can be recommended.

Table 1.4. Effect of a Latin square design on the reaction of ten IITA yam parental lines to S.

bradys eight weeks after storage (Inoculated with ca 2,500 S. bradys [50 g infected yam

peelings])

*Mean dry rot index Mean no. S. bradys/ 5 g tuber peelings

Naturally dArtificially Naturally infested soil Artificially infested Soil

Yam Infested Infested kTrans-

formed s.e

Trans-

Formed s.e Accession

Soil Soil No. No.

TDr 131a 0.3 3.0 80.0 5.7 6.8 1481.4 37.9 3.5

TDr 89/01438b 3.0 3.0 845.7 23.7 8.0 800.6 26.4 3.5

TDr 93-28a 1.6 3.0 367.0 16.1 6.0 716.2 25.1 3.5

TDr 93-72a 2.6 3.0 741.6 25.2 6.0 395.6 19.1 3.5

TDr 89/02677b 2.6 3.0 1101.6 31.2 6.0 849.0 27.0 3.5

TDr 93-82a 2.5 3.0 521.3 19.7 6.8 447.5 20.6 3.9

TDr 93-1a 1.8 3.0 724.0 23.5 6.0 146.0 11.8 3.5

TDr 93-31a 2.5 3.0 641.3 22.6 6.8 147.7 11.9 4.6

TDr 93-83a 1.0 3.0 431.3 17.5 6.8 86.3 8.0 3.5

TDr 93-2a 2.2 3.0 954.0 26.4 6.0 435.0 19.5 3.5

CV 50.9 0.0 62.7 36.3 cRoot MSE 1.0 0.0 13.5 7.8

*Average of 5 replicates, s.e: Standard error. TDr: Dioscorea rotundata, alandrace,

bBreeder’s line

kSquare root (Mean + 0.5) and SAS adjusted for missing data.

CRoot mean square error.

dNaturally

infested soil plus chopped S. bradys infested yam peelings.

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Table 1.5. Effect of an Alpha array design on the reaction of ten IITA yam parental

lines to S. bradys eight weeks after storage (Inoculated with ca 2,500 S. bradys [50 g

infected yam peelings]) Mean no. S. bradys/ 5 g tuber peelings

*Mean dry rot index Naturally infested soil Artificially infested soil

Yam Naturally dArtificially

kTrans-

formed s.e

Trans-

formed s.e Accession infested soil infested soil No. No.

TDr 131a 1.6 3.0 375.8 16.7 5.6 1198.3 26.6 7.9

TDr89/01438b 1.9 3.0 460.4 16.9 4.7 470.0 20.1 7.9

TDr 93-28a 1.3 3.0 288.3 15.4 7.5 898.7 25.4 5.0

TDr 93-72a 2.1 2.3 759.3 23.9 4.7 178.3 11.8 8.0

TDr 89/ 02677b 2.3 2.7 739.3 26.6 5.1 329.0 18.0 8.0

TDr 93-82a 1.5 3.0 590.0 17.6 9.3 570.6 22.1 4.6

TDr 93-1a 2.3 2.3 137.3 10.3 7.6 164.7 10.5 5.4

TDr 93-31a 1.3 3.0 251.7 11.7 7.5 265.0 14.8 5.5

TDr 93-83a 0.9 2.3 147.4 8.8 4.4 393.3 19.0 8.0

TDr 93-2a 0.0 2.3 20.0 3.1 7.6 318.5 16.4 6.8

CV 70.6 29.6 72.9 67.5 cRoot MSE 1.1 0.8 11.8 12.6

*Average of 10 replicates, s.e: Standard error. TDr: Dioscorea rotundata, aLandrace,

bBreeder’s

line. k

Square root (Mean + 0.5) and SAS adjusted for missing data. CRoot mean square error.

dNaturally infested soil plus chopped S. bradys infested yam peelings.

STUDIES ON THE INVASION AND POPULATION DYNAMICS OF S.BRADYS IN YAM TUBERS.

Nematode population at harvest and subsequent dry rot symptoms ranged from 0 to 2.8 and

0 to 560 respectively. At 8 weeks after harvest, the range was 0 to 3 for dry rot and 0 to

800 for number of nematodes. Generally, symptoms and nematode population density

increased with time in the susceptible yam accessions, except in TK106 which had little or

no living tissue left for invasion. There was no change in dry rot symptom expression and

invasion in the resistant yam accession, D. dumetorum (Table 1.6). In susceptible yam

tubers the S. bradys had invaded at harvest and continued to reproduce in storage. This

work cannot detail the mechanism of resistance in D. dumetorum, but it is likely that post-

infectional characteristics in the yam tuber, confer resistance. It is less likely that

nematodes are completely prevented from entering the D. dumetorum tuber.

Table 1.6. Invasion and population dynamics of S. bradys in yam tubers

(Means of 6 replicates and inoculated with ca 2,500 S. bradys [50 g yam peelings])

Yam accession

(all D. rotundata

except otherwise stated)

Dry rot score

Mean no. S. bradys/5 g tuber peelings

Harvest After 8 wk Harvest After 8 wk

TDr 89/01438 1.7 1.8 210.0 490.0

CR1/0051 2.8 3.0 180.0 800.0

D. dumetorum 0.0 0.0 0.0 0.0

TDr 131 2.2 2.4 60.0 680.0

TK 106 2.0 2.7 560.0 60.0

13

INVESTIGATION OF THE INOCULUM TYPE AND SIZE OF S. BRADYS

In most cases, the S. bradys infected yam peelings recorded higher nematode numbers than

with the nematode in water suspension (Table 1.7). There was no significant difference

between the inoculum size in peelings however, nematode numbers increased from the

lowest weight of 1g to the highest of 100 g peelings. Nematode suspensions did not give

consistent reproduction of S. bradys in both yam accessions. The coefficient of variation

(CV) was high for suspension, for example, 68.8 and 80.2 for number of S. bradys in roots

and tubers as against 46.4 to 67.8 for peelings respectively. Based on the results, S. bradys

infected peelings are a better source of inoculum, if available, for screening for nematode

resistance in yam.

Table 1.7. Efficiency of inoculum type and size of S. bradys in resistance screening Mean tuber

Cracking

Index

Mean no. S. bradys/ 5 g tissue

Yam

accession

Inoculum Tuber

peelings

Type Size Roots Range Range

TDa 294 Peels 1.0 g 0.5 (0.6) 306.0 65.0-675.0 213.0 0.0-820.0

TDa 294 Suspension 120.0 1.0 (0.0) 9.0 0.0-35.0 0.0 0.0-0.0

TDa 294 Peels 25.0 g 0.0 (0.0) 429.0 150.0-707.0 85.0 60.0-100.0

TDa 294 Suspension 2800.0 0.3 (0.6) 119.0 15.0-220.0 11.0 0.0-30.0

TDa 294 Peels 100.0 g 0.7 (0.6) 461.0 287.0-635.0 368.0 55.0-680.0

TDa 294 Suspension 10000.0 1.0 (0.0) 132.0 0.0-215.0 34.0 0.0-90.0

TDa 92-2 Peels 1.0 g 1.0 (0.0) 136.0 15.0-310.0 95.0 30.0-245.0

TDa 92-2 Suspension 120.0 1.5 (0.6) 3.0 0.0-10.0 16.0 10.0-25.0

TDa 92-2 Peels 25.0 g 0.1 (0.4) 421.0 116.0-1160.0 430.0 80.0-1413.0

TDa 92-2 Suspension 2800.0 1.0 (0.0) 30.0 0.0-105.0 32.0 0.0-85.0

TDa 92-2 Peels 100.0 g 0.3 (0.5) 437.0 0.0-1080.0 518.0 20.0-890.0

TDa 92-2 Suspension 10000.0 0.7 (0.6) 55.0 0.0-155.0 71.0 0.0-190.0

Standard deviation in parentheses. Mean of 5 replicates.

INFLUENCE OF REPLICATION ON PRECISION IN ESTIMATING QUANTITATIVE DIFFERENCES

IN SUSCEPTIBILITY OF YAM TO NEMATODES

The number of S. bradys/5g tissue ranged from 135 to 929 for roots and 1 to 55 for tubers

for TDr 87/00571 and 265 to 1889 for roots and 4 to 40 for tubers for TDr 87/00211 (Table

1.8), with significant differences (P=0.05) between the replications. Increasing replication

improves the precision of quantitative differences in susceptibility. In practice, practical

considerations will govern the level of replication, but the results suggest that greater than

5 replicates give better separation of quantitative differences in susceptibility in yam to S.

bradys.

14

Table 1.8. Influence of replication on quantitative difference in susceptibility of yam to S.

bradys (Inoculated with 1700 S. bradys)

Mean tuber

cracking

index

Mean no. S. bradys/ 5 g tissue

Replicate

number

Yam

accession

Tuber

peelings

s.e Roots s.e s.e

3 TDr 87/00211 1.0 (0.0) 0.3 135.0 (82.9) 6.8 2.0 (4.0) 27.5

5 TDr 87/00211 1.4 (0.6) 0.2 343.0 (164.9) 5.2 1.0 (0.6) 21.3

13 TDr 87/00211 0.5 (0.5) 0.1 929.0 (642.3) 3.3 55.0 (58.4) 13.2

3 TDr 87/00571 1.0 (0.0) 0.4 265.0 (243.0) 12.2 4.0 (4.9) 1.9

5 TDr 87/00571 2.3 (1.2) 0.3 956.0 (484.5) 9.4 27.0 (23.1) 1.5

13 TDr 87/00571 2.0 (0.8) 0.2 1889.0 (1715.3) 7.0 40.0 (38.6) 0.9

Standard deviation in parenthesis. s.e : standard error

15

OUTPUT 2 YAM GERMPLASM SCREENED FOR RESISTANCE TO

NEMATODES

PRELIMINARY SCREENING FOR NEMATODE RESISTANCE OF IITA YAM

The highest number of nematodes recorded was 4,500/5 g for S. bradys and 250/5 g for

Meloidogyne species. S. bradys was the most prevalent nematode occurring in 46% of

accessions. 10.2 % and 0.3 % of the accessions were infested with Meloidogyne and

Pratylenchus species respectively. No linear relationship was observed between symptom

ratings and nematode population densities in tubers for either nematode. Based on the

number of nematodes per unit tissue selection criteria, the following number of accessions

were placed in each reaction category: 0-10 = Resistant (R), 11-25 = Partially resistant

(PR), 26–50 = Moderately susceptible (MS), > 50 = Susceptible (S) and Sensitive = severe

symptom and low nematode density.

Table 2.1. IITA yam accessions grouped under reaction categories

No. of accessions for nematodes reactions

S. bradys Meloidogyne species Pratylenchus

Reaction

category D. rotundata D. alata D. rotundata D. alata Yams

R 160.0 13.0 202.0 18.0 0.0

PR 43.0 2.0 43.0 0.0 0.0

MS 48.0 0.0 24.0 0.0 2.0

S 52.0 3.0 9.0 0.0 6.0

Sensitive 3.0 0.0 28.0 0.0 0.0

SCREENING OF SELECTED GENOTYPES OF IITA YAM ACCESSIONS FOR RESISTANCE TO S.

BRADYS AND MELOIDOGYNE INCOGNITA, NIGERIA.

In field screening all genotypes selected were susceptible to S. bradys, but there were

significant differences (P<0.01) between accessions. There was a strong linear relationship

between dry rot symptoms and S. bradys population. The number of S. bradys/5 g ranged

from 0 to 1,825. In most cases, higher dry rot scores were associated with higher the

numbers of S. bradys (Table 2.2).

16

Table 2.2. Selected IITA yam accessions and their reaction to S. bradys

(Mean of unequal number of replicates (3-12) and inoculated with about 800 S. bradys

(50g infected yam peels)

Yam accession

Dry rot index (0-3)

Harvest After 8 weeks No. S. bradys/5 g after 8 weeks

TDa 92-2 1.0 2.9 1825.0

TDa 95-92 1.6 3.0 1167.0

TDr 93-91 0.8 2.2 1086.0

TDa 96-12 1.3 2.8 1048.0

TDa 297 1.0 2.8 840.0

TDr 93-22 2.0 2.3 529.0

IN94R-23 0.5 2.2 488.0

TDr 131 1.0 2.6 427.0

TDa 93-36 1.8 2.9 408.0

TDa 96-5 1.3 2.5 393.0

TDr 93-49 1.2 2.3 327.0

TDr 93-82 0.7 2.7 285.0

IN94R-5 0.5 2 0 260.0

TDr 93-74 1.2 2.2 211.0

TDa 95-27 2.0 2.5 209.0

TDr 93-9 3.0 3.0 160.0

TDr 93-72 2.0 3.0 128.0

TDr 93-1 1.0 3.0 105.0

TDr 93-39 0.0 0.0 0.0

In general, D. alata (TDa) had higher population densities of S. bradys than D. rotundata

(TDr) ranging from 209 to 1825 and 0 to 1086 respectively. All IITA D. alata and D.

rotundata accessions screened are susceptible to S. bradys.

17

Table 2.3. Reaction of IITA yam accessions to Meloidogyne incognita in pot trials

(Means of 8 replicates and inoculated with about 400 Meloidogyne juveniles [J2])

Yam accession (all D. rotundata

except otherwise specified)

Tuber galling

(0-3)

Root galling

(0-3)

Meloidogyne J2/5 g tissue

Roots Tuber

1010 1.0 0.4 22.0 23.0

D. mangenotiana (Guinea forest) 0.1 0.6 268.0 13.0

TDr 93-82 0.5 0.9 5.0 6.0

TDr 179 0.4 0.3 4.0 4.0

TDr 89/02565 0.4 0.4 12.0 3. 0

KK102 1.0 0.8 15.0 3.0

TDr 131 0.4 0.4 3.0 2.0

TDr 89/02677 0.8 0.3 47.0 2.0

TDr 93-25 0.5 0.6 16.0 1.0

2008 0.4 0.6 13.0 0.0

1024 0.3 0.1 3.0 0.0

1020 0.5 0.8 2.0 0.0

1009Tm101 0.1 0.3 0.0 0.0

BP 129 0.1 0.3 3.0 0.0

Ka 120 0.1 1.0 8.0 0.0

Yb 114 0.1 1.0 0.0 0.0

KP 106 0.0 0.0 9.0 0.0

NF 104 0.3 0.6 3.0 0.0

TDr 93-1 0.1 0.9 2.0 0.0

TDr 93-28 0.3 0.5 3.0 0.0

TDr 93-31 0.0 0.3 0.0 0.0

TDr 93-47 0.0 0.3 0.0 0.0

TDr 93-72 0.4 0.3 3.0 0.0

TDr 93-74 0.4 0.4 0.0 0.0

TDr 95-78 0.1 0.5 5.0 0.0

TDr 93-79 0.3 0.4 8.0 0.0

TDr 95-83 0.3 0.6 53.0 0.0

TDr 93-89 0.1 0.0 2.0 0.0

TDr 87/00158 0.1 0.9 0.0 0.0

TDr 89/01438 0.0 0.1 0.0 0.0

TDr 89/02461 0.8 0.1 0.0 0.0

TDr 89/02494 0.4 0.8 10.0 0.0

IN94R-5 0.0 0.4 0.0 0.0

TDr 95-158 0.3 0.8 0.0 0.0

IN94R-102 0.0 0.4 1.0 0.0

D. praehensilis (Moyenne in Guinea) 0.1 0.8 0.0 0.0

BP128 0.3 0.6 7.0 0.0

Baniakpa 0.0 0.1 0.0 0.0

D. praehensilis (Guinea forest) 0.0 0.3 0.0 0.0

D. Praehensilis (Benin Republic) 0.3 0.4 0.0 0.0

Standard deviation 0.6 0.5 0.9 0.1

Analysis of variance showed highly significant differences (P = 0.01) between yam

accesssions. The mean number of Meloidogyne incognita ranged from 0 to 268/ 5g roots

and 0 to 23/5 g tuber (Table 2.3). There was no apparent correlation between galling in

roots and tubers and numbers of nematodes in root and tubers. There was no linear

relationship between susceptibility in roots and tubers. These are results are difficult to

interpret especially regarding classification because of the poor reaction of the yams to M.

18

incognita. This is probably because of the high temperatures recorded in the glasshouse

resulting in killing of the nematodes or making them impotent.

SCREENING OF GHANAIAN LANDRACES FOR RESISTANCE TO S. BRADYS AND

MELOIDOGYNE SPECIES

Based on the farmer participatory appraisal, 40 Ghanaian yam landraces were screened.

There were highly significant differences (P = 0.01) between yam varieties. There was

weight reduction in the stored yam tubers, and dry rot scores also increased within the

storage period. There was a linear relationship between the weight reduction in tubers

infected with S. bradys and the increased dry rot symptoms. It is noteworthy that the

severity of both dry rot and root knot nematode galls increased during storage. This

implies that both species continue to multiply in storage. Gall scores did not correlate with

tuber weight loss and confirm farmer perceptions that root knot nematode is a less

important nematode in terms of crop loss. Dry rot and S. bradys population density

revealed a strong negative correlation (r =0.42, P = 0.01). S. bradys counts at four weeks

ranged from 0 to 951/5 g and from 0 to 675/5 g at 11 weeks after harvest (Table 2.4).

Again, severely infected yam tubers recorded high nematode populations. However, some

severely infected tubers had low S. bradys population probably because these tubers were

either dried out or completely destroyed by dry and wet rot with very little or no living

tissue remaining. Ten contrasting varieties were selected for further studies and results

confirmed that D. dumentorum var. nkanfo and D. cayenensis var. Afun are resistant to S.

bradys. D. dumentorum var. nkanfo is also resistant to Meloidogyne incognita.

19

Table 2.4. Ghanaian landraces reaction to S. bradys resistance, Crops Research

Institute.

(Field trial, 4 replicates and inoculated with about 6,000 S. bradys (50 g infected yam

peelings)

Yam varieties Dry rot index (0-3) No. S. bradys/5 g tuber peelings

Harvest 4 wk 11 wk 4 wk 11 wk

Yeremma 1.5 (1.0) 1.5 (0.6) 2.3 (0.5) 1073.0 1050.0

Saabiri 0.5 (0.6) 1.3 (1.0) 1.8 (0.5) 405.0 988.0

Sante 0.5 (1.0) 1.3 (0.5) 2.0 (0.8) 870.0 880.0

Matches 1.5 (0.6) 2.0 (0.0) 2.3 (0.5) 518.0 869.0

Afi 0.7 (1.2) 2.0 (0.0) 2.3 (0.6) 646.0 863.0

Mmrefi 0.5 (0.6) 0.8 (1.0) 1.3 (1.0) 430.0 817.0

Nigeria 1.0 (0.8) 1.5 (0.6) 2.0 (0.0) 1004.0 804.0

Afasie-Kwandwo 0.5 (0.6) 1.0 (0.0) 2.0 (0.0) 398.0 749.0

Datordi 1.8 (0.5) 2 .0 (0.0) 2.5 (0.6) 933.0 709.0

Accra 2.0 (0.0 2.0 (0.0) 2.3 (0.5) 821.0 504.0

Tempe 1.3 (1.0) 1.8 (0.6) 2.0 (0.8) 470.0 468.0

Nsoadansi 1.0 (0.0 1.5 (0.6) 2.0 (0.0) 1125.0 419.0

Puna 2.3 (0.5) 2.3 (0.5) 2.5 (0.6) 371.0 416.0

Abrewa nwo 1.0 (1.2)* 2.3 (0.5) 2.5 (0.6) 598.0 396.0

Kyemogo 0.8 (0.1) 1.0 (0.0) 2.3 (0.5) 327.0 379.0

Sanyata 2.3 (1.0) 2.3 (1.0) 2.8 (0.5) 541.0 331.0

Kpirindwo 2.3 (0.6) 1.7 (0.6) 2.3 (0.6) 482.0 322.0

Akaba 1.8 (1.3) 2.5 (0.6) 2.5 (1.0) 780.0 320.0

Zong 1.5 (0.6) 2.0 (0.0) 2.8 (0.8) 761.0 308.0

Ziglangbo 0.8 (1.0) 1.8 (0.5) 1.5 (0.6) 525.0 275.0

Kwaa-Asamoah 1.3 (1.0) 2.0 (0.8) 2.0 (0.8) 132.0 255.0

Serwaah 2.3 (0.6) 2.3 (0.6) 3.0 (0.0) 525.0 232.0

Dakpam 1.3 (1.0) 1.0 (1.2) 1.5 (1.3) 194.0 229.0

Limor 2.3 (0.5) 2.5 (0.6) 2.8 (0.5) 478.0 171.0

Moninyoli 2.0 (0.8) 1.8 (0.5) 2.5 (1.0) 647.0 153.0

Dakorba 2.0 (0.8) 2.0 (0.0) 2.5 (0.6) 315.0 136.0

Lili 1.0 (0.0) 1.5 (1.0) 2.3 (1.0) 518.0 125.0

Muchumudu 2.0 (0.8) 1.8 (0.5) 2.8 (0.5) 516.0 109.0

Denteh 1.3 (1.0) 1.5 (0.6) 2.0 (1.2) 245.0 96.0

Kyire-Kumasi 1.3 (1.2) 1.7 (0.6) 2.0 (1.0) 368.0 95.0

Kpiringa 2.0 (0.0) 2.3 (0.5) 3.0 (0.0) 467.0 93.0

Chenchito 2.5 (0.5) 2.5 (0.5) 3.0 (0.0) 790.0 91.0

Agyaasi 1.3 (1.0) 1.5 (0.6) 2.3 (1.0) 250.0 86.0

Sono bayere 1.8 (1.0) 2.0 (0.0) 2.5 (0.6) 732.0 69.0

Labarko 2.0 (1.7) 2.3 (1.2) 2.7 (0.6) 71.0 68.0

Adi-amaaba 0.5 (0.6) 1.0 (0.0) 0.8 (0.5) 829.0 45.0

Fugla 1.5 (1.0) 1.5 (0.6) 2.8 (0.5) 226.0 38.0

Tela 0.8 (1.0) 1.3 (1.0) 1.3 (1.0) 380.0 28.0

Afun 0.3 (0.5) 0.3 (0.5) 0.3 (0.5) 3.0 5.0

Nkanfo 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 0.0

*Standard deviation in parentheses.

20

Pot Screening of Ghanaian landraces for resistance to S. bradys

The number of S. bradys/5 g roots ranged from 180 to 1,569 and for S. bradys/ 5 g tubers

from 5 to 274 (Table 2.5). There were highly significant differences (P = 0.01) between the

yam varieties. There were more nematodes in roots than in tubers however, there was no

linear correlation between them. There was also no correlation between dry rot and

nematodes population in tubers. Cluster analysis and Duncan Multiple Range Test showed

that D. dumetorum var. Nkanfo is resistant and this agreed with the field experiments.

Table 2.5. Reaction of selected Ghanaian landraces to S. bradys in pots

(Means of 4-6 replicates and inoculated with 1,400 S. bradys (55 g yam peels)

Yam varieties

Dry rot of

tubers (0-3)

No. S. bradys/5 g tissue

Roots Tuber

Chenchito 1.0 806.0 274.0

Lili 1.7 1281.0 261.0

Kyerekumasi 2.4 225.0 117.0

D. bulbifera 1.0 916.0 75.0

D. esculenta 0.0 1569.0 73.0

D. cayenensis var. Afun 1.6 182.0 7.1

D. dumetorum var. Nkanfo 0.5 180.0 5.0

Pot Screening of Ghanaian landraces for resistance to Meloidogyne species

The number of juveniles of Meloidogyne incognita /5 g roots ranged from 6 to 1104 and

from 6 to 170/5 g tubers (Table 2.6). There were highly significant differences (P = 0.01)

between the yam varieties. Nkanfo recorded the lowest number of nematodes in roots and

tubers as well as coefficient of variation (CV). There was no linear correlation between

tuber galling and population of J2. Not surprisingly there were several significant

correlations, e.g. root galling and J2 population in roots (r = 0.53, P = 0.05); galling and

number of eggs in tubers (r = 0.62, P = 0.01); tuber galling and J3 population in tubers (r =

0.49, P = 0.05) and tuber galling and number of females in tubers (r = 0.71, P = 0.01).

Cluster analysis and Duncan Multiple Range Test revealed that D. dumetorum var. Nkanfo

is resistant and D. bulbifera and Chenchito, highly susceptible. This result agreed with

field data.

21

Table 2.6. Reaction of selected Ghanaian landraces to Meloidogyne incognita in pots

(Means of 4-6 replicates and inoculated with 10000 Meloidogyne J2 (10 g galled tomato

roots)

Yam

varieties

Tuber

galling

(0-3)

Root

galling

(0-3)

J2/5 g

roots

Eggs/5 g

tuber

J2/5 g

tuber

J3/5 g

tuber

Females/5 g

tuber

Nkanfo 1.0 0.5 6.0 3.0 6.0 0.0 0.0

Afun 1.6 1.2 58.0 69.0 36.0 19.0 16.0

D. bulbifera 0.5 1.3 1104.0 1 0.0 170.0 0.0 3.0

Yeremma 1.6 2.0 821.0 55.0 13.0 10.0 8.0

Chenchito 2.4 2.4 634.0 322.0 99.0 162.0 60.0

GLASSHOUSE INVESTIGATION OF THE VARIATION IN SUSCEPTIBILITY OF YAM SPECIES TO

S. BRADYS, UK.

Table 2.7. Reaction of S. bradys in roots and tubers of yam species

(Mean of 4 replicates and inoculated with 500 S. bradys)

Mean no. S. bradys/5 g tissue

Yam species Roots Tuber

D. dumetorum 10630.0 800.0

D. abyssinica 1035.0 625.0

D. rotundata 89/01892 950.0 255.0

D. rotundata 91/00658 295.0 225.0

D. preussii 140.0 5.0

D. togoensis 140.0 0.0

There were no significant differences between treatments. The mean number of S. bradys

ranged from 140 to 10,630/5 g roots and 0 to 800/5 g tuber (Table 2.7). Again, there were

more nematodes in the roots than tubers. None of the entries exhibited symptoms of

nematode injury, furthermore there was no apparent relationship between susceptibility in

roots and tubers, suggesting that susceptibility in roots of yams is not clearly linked to the

susceptibility of tubers. D. togoensis and D. preussii were found to be potentially resistant

and D. dumetorum and D. abyssinica were highly susceptible. The results of this test

contrast sharply with others indicating that D. dumetorum has resistance to both S. bradys

and Meloidogyne incognita. This appears to indicate that resistance to nematodes is not

likely to be species specific.

FIELD SCREENING OF OPEN-POLLINATED SEEDLING POPULATION FOR S. BRADYS

RESISTANCE.

The dry rot symptoms ranged between 0 to 3 while the number of S. bradys was between 0

to 1,985/ 5 g tuber. Cluster analysis and Duncan Multiple Range test for dry rot scores and

the number of S. bradys/5 g tuber showed that 6 % of the genotypes are potentially

resistant whilst 94 % are highly susceptible.

22

POT SCREENING OF YAM PARENTAL LINES FOR RESISTANCE TO S. BRADYS

All the parental lines were susceptible to S. bradys and there were no significant

differences between them. The number of S. bradys/5 g roots ranged from 507 to 1,518

and for number of S. bradys/5 g from 9 to 138 (Table 2.8).

Table 2.8. Variation in susceptibility of yam parental lines to S. bradys

(Means of 4-7 replicates and inoculated with about 1,700 S. bradys)

Tuber cracking

(0-3)

S. bradys/5 g tissue

Yam parental lines Roots Tuber

TDr 93-1 1.1 1479.o 18.0

TDr 93-2 1.6 1225.0 104.0

TDa 92-2 0.7 792.0 42.0

TDa 95/00328 1.4 680.0 138.0

TDr 87/00571 1.6 1109.0 24.0

TDa 87/01091 1.0 716.0 13.0

TDa 95-310 1.0 507.0 9.0

TDa 85/00257 1.0 835.0 33.0

TDr 93-50 2.0 1518.0 59.0

TDr 87/00211 0.9 558.0 24.0

All the parental lines are susceptible in roots and tuber to S. bradys. However, variation in

susceptibility among the lines was observed. There was no correlation between the number

of S. bradys in roots and that in tubers.

POT SCREENING OF PARENTS AND PROGENIES OF INTRASPECIFIC HYBRIDISATIONS FOR

RESISTANCE TO NEMATODES

There were no significant differences between the genotypes regarding the number of S.

bradys in tubers. Again, no correlation was found between the numbers of nematodes in

roots and that in tubers. All the hybrids and their parents in the screen were susceptible to

S. bradys although there was quantitative variation in susceptibility. Selected contrasting

hybrids screened on the field were all susceptible and agreed with the pot trial. This

suggests that resistance of yam to S. bradys is not likely to be controlled by recessive

genes. The number of S. bradys/5 g root ranged from 16 to 4935 and that of S. bradys/5g

tuber from 0 to 259 (Table 2.9). Higher numbers of the nematodes were again found in

roots than in tubers.

23

Table 2.9. Reaction of yam intraspecific hybrids to S. bradys

(Means of 4–8 replicates inoculated with about 780 S. bradys)

Parents and hybrids

Tuber

cracking (0-3)

Mean no. S. bradys/5 g tissue

Roots Tuber

CR1/0096 0.7 450.0 16.0

CR1/0051 0.8 1098.0 50.0

CR/002 1.0 1795.0 42.0

CR1/0036 1.3 222.0 17.0

CR1/0058 1.0 1590.0 123.0

CR1/00102 1.0 470.0 8.0

CR1/00192 1.5 1036.0 238.0

CR1/0027 2.1 4935.0 259.0

CR1/0020 0.6 132.0 1.0

CR1/0029 1.0 781.0 11.0

CR1/00193 0.5 583.0 3.0

CR1/OO92 1.0 1658.0 16.0

CR1/0022 0.4 210.0 12.0

CR1/00172 1.8 1464.0 116.0

CR1/0095 1.5 1301.0 25.0

CR1/001 3.0 1362.0 180.0

CR1/0034 1.0 222.0 21.0

CR1/0048 2.0 1491.0 186.0

CR1/0065 1.3 237.0 12.0

CR1/0098 1.8 959.0 107.0

CR1/0044 1.3 259.0 25.0

CR1/0039 1.4 67.0 18.0

CR1/0024 0.5 1742.0 25.0

CR1/0091 1.8 1293.0 107.0

CR1/00146 0.7 890.0 23.0

CR1/0046 2.8 662.0 66.0

CR1/0025 0.5 1297.0 8.0

CR1/0038 1.3 368.0 38.0

CR1/OO64 1.2 325.0 58.0

CRI/0041 1.0 1169.0 30.0

CR1/00113 1.4 936.0 134.0

CR1/00130 0.3 209.0 23.0

CR1/0075 1.3 282.0 34.0

CR1/00138 1.2 728.0 91.0

CR1/0056 0.3 904.0 7.0

CR1/0035 1.3 40.0. 38.0

CR1/00173 1.3 316.0 83.0

CR1/0069 1.3 979.0 29.0

CR1/00129 0.8 208.0 7.0

CR1/0054 1.0 4689.0 152.0

CR1/0016 1.0 1288.0 0.0

CR1/0053 1.4 1060.0 31.0

CR1/00101 1.0 55.0 68.0

24

Table 2.9 (cont.)

Parents and hybrids

Tuber

cracking (0-3)

Mean no. S. bradys/5 g tissue

Roots Tuber

CR1/0040 0.6 1217.0 13.0

CR1/0037 1.0 536.0 8.0

CR1/0052 1.0 1681.0 51.0

CR1/0028 0.3 650.0 30.0

CR1/00105 1.3 304.0 4.0

CR1/00143 3.0 771.0 50.0

CR1/00126 1.0 175.0 45.0

CR1/0074 1.5 1161.0 147.0

CR1/00106 1.0 624.0 26.0

CR1/0093 1.0 610.0 71.0

CR1/0011 0.8 1745.0 42.0

87/00571 2.0 1194.0 14.0

87/00211 0.7 735.0 22.0

CR1/00139 2.7 446.0 116.0

CR1/0030 1.8 2741.0 115.0

CR1/0088 1.0 16.0 130.0

The parent (TDr 87/00211) was highly susceptible to Meloidogyne incognita (Table 2.10).

There was variation in susceptibility between hybrids, but very careful and precise

screening and selection would be required to utilise this variation with confidence.

Procedurally, it is appropriate to use all stages of Meloidogyne species for assessment of

susceptibility in yam tubers as there were significant differences (P = 0.01) between egg,

J3 and female counts in tubers.

Table 2.10. Reaction of yam intraspecific hybrids to Meloidogyne incognita

(Means of 4-8 replicates and inoculated with about 400 Meloidogyne J2s)

Yam hybrid

and parent

Tuber

galling

(0-3)

Root

Galling

(0-3)

Root

knot J2/5

g root

Root

knot

eggs/5 g

tuber

Root knot

J2/5 g

tuber

Root

knot

J3/5 g

tuber

Root

knot

female/5

g tuber

CR1/0090 2.0 1.7 1753.0 2042.0 94.0 131.0 388.0

TDr 87/00211 3.0 2.0 8808.0 0.0 74.0 124.0 173.0

CR1/0042 2.3 1.3 2423.0 280.0 93.0 25.0 141.0

CR1/0021 2.0 1.3 5404.0 84.0 33.0 31.0 132.0

CR1/00147 1.8 0.8 8780.0 89.0 31.0 56.0 128.0

CR1/00107 3.0 1.0 2896.0 38.0 8.0 13.0 96.0

CR1/00112 2.0 0.5 3994.0 83.0 24.0 13.0 65.0

CR1/0080 2.0 0.5 1108.0 0.0 4.0 17.0 56.0

CR1/0062 0.5 0.5 1172.0 15.0 11.0 12.0 49.0

CR1/0073 0.0 0.5 4621.0 65.0 0.0 16.0 49.0

CR1/0070 2.0 0.7 2436.0 15.0 2.0 7.0 46.0

CR1/0079 1.4 1.0 2263.0 15.0 0.0 20.0 42.0

CR1/0031 1.2 0.7 1321.0 39.0 17.0 3.0 32.0

CR1/0051 2.0 1.0 1033.0 0.0 3.0 3.0 24.0

25

Table 2.10 (cont.)

CR1/0033 1.0 0.2 1508.0 16.0 10.0 3.0 23.0

CR1/003 1.3 1.3 2710.0 12.0 4.0 7.0 20.0

CR1/00132 1.0 0.3 778.0 97.0 8.0 1.0 16.0

CR1/007 0.5 0.5 328.0 2.0 2.0 2.0 12.0

CR1/00118 1.5 0.5 674.0 9.0 21.0 15.0 12.0

CR1/0089 0.8 0.8 1293.0 37.0 15.0 2.0 10.0

CR1/00111 0.8 0.3 142.0 35.0 5.0 3.0 9.0

CR1/00123 1.8 0.3 690.0 15.0 10.0 0.0 8.0

CR1/00128 1.0 0.7 678.0 10.0 24.0 0.0 7.0

CR1/0077 1.0 1.0 1549.0 2.0 0.0 2.0 5.0

CR1/006 2.0 0.5 410.0 0.0 0.0 0.0 0.0

Field Screening of Cellular and Molecular technologies laboratory (IITA) accessions

and interspecific hybrids for resistance to S. bradys

The population density of S. bradys ranged from 17 to 2,502/5 g yam peelings (Table

2.11). There were highly significant differences (P = 0.01) between the accessions. There

was correlation (r= 0.39, P<0.01) between S. bradys population and dry rot symptom

expression. There was also correlation between all the plant characteristics like softness,

crack and population density. All the interspecific hybrids were highly susceptible to the

nematode. The wild accession, D. dumetorum was found to be resistant to S. bradys.

Table 2.11. Reaction of yam interspecific hybrids and accessions to S. bradys (Mean of 10 replicates after 8 weeks and inoculated with 2,500 S. bradys (50 g infected peels)

Yam accession

Tuber

cracking

(0-3)

Tuber

softness

(0-3)

Dry rot

index

(0-3)

Mean no. S. bradys/5 g

tuber peelings

TDr 98/00216 2.9 1.1 3.0 2502.0

TDr 89/00217 2.8 1.0 3.0 1156.0

TDr 97/01316 3.0 0.5 3.0 1105.0

TDr 97/01314 2.5 0.7 2.5 1028.0

Kappe 138 2.3 0.6 2.6 827.0

TDr 97/01306 2.5 0.9 2.6 654.0

TDr 97/01275 2.4 0.4 2.9 189.0

BP 128 2.4 0.6 2.9 134.0

D. dumetorum 0.0 0.0 0.0 17.0

Field screening of yam intraspecific hybrids for resistance to S. bradys

S. bradys population ranged for 373 to 1,054/5 g yam peelings and dry rot ranged from 2.6

to 3 (Table 2.12). There were no significant differences between clones. This implies that

all the clones are susceptible to S. bradys. The parents of the hybrids were also susceptible.

26

Table 2.12 Reaction of intraspecific hybrids to S. bradys 8 weeks after harvest

(Mean of 10 replicates and inoculated with 2,500 S. bradys (50g infected yam peels)

Yam hybrids

Tuber

cracking

(0-3)

Tuber

softness

(0-3)

Dry rot

(0-3)

Mean no. S. bradys/5g

CR1/0083 2.2 1.0 3.0 628.0

CR1/0074 2.1 0.9 2.9 696.0

CR1/0057 2.2 1.5 3.0 373.0

CR1/0024 2.1 1.1 3.0 746.0

CR1/0051 2.6 1.4 3.0 1054.0

CR1/0036 2.4 1.0 3.0 1023.0

CR1/0099 2.3 1.6 3.0 500.0

CR1/0075 1.5 1.2 2.6 973.0

CR1/0037 2.6 0.9 3.0 617.0

CR1/00143 2.7 1.0 3.0 472.0

OUTPUT 3 NEMATODE VARIABILITY ON YAM KNOWN

PARTICIPATORY RURAL APPRAISAL: NEMATODE PESTS PROBLEMS OF YAM IN GHANA

The symptoms of nematode injury were clearly identified by farmers. Most farmers were

very familiar with the dry rot of tubers caused by migratory endoparasitic nematodes. In

parts of the country, more than 90% of farmers had local names for the disease symptoms

e.g. nkronsa nkronsa or edwie (rashes). Farmers could readily identify tubers with dry rot

symptoms and these were rejected at planting or consumed early. Farmers estimated losses

from dry rot to be 21% (0-100) in the Forest zone and 30% (2-100) in the Savannah.

Losses from root knot nematode were estimated as 11% (0-40) in the forest and zero in the

savannah (Table 3.1). The prevalence of the symptoms was 83% of sites in the forest zone

and 100% in the savannah (Table 3.2). The dominant component crops are also shown. At

all sites, dry rot symptoms were associated with S. bradys and galling with Meloidogyne

incognita. In most years farmers grew 10-15 yam varieties mostly of Dioscorea rotundata,

but also D. alata, D. cayenensis, D. dumetorum and D. bulbifera. All were thought by

farmers to be susceptible to dry rot, but some varieties e.g. D. rotundata var. Lili and

species of D. alata were said to store better. Traditional yam varieties were often described

to be free of dry rot. There can be few cases where a nematode problem is culturally so

important and so well understood by farmers. This suggests that farmers would adopt a

resistant variety, if such is identified or developed. However, farmers grow a large number

of yam varieties for different reasons like taste and yield, so any new variety should fit this

background.

Table 3.1. Farmer’s estimates of the proportion of tubers affected by dry rot and root

knot nematode in Ghana

Agroecological zone Dry rot disease (%) Root knot galling (%)

Mean Range Mean Range

Forest-transitional 21 0-100 11 0-40

Guinea-Savannah 30 2-100 0 0

27

Table 3.2 Prevalence of dry rot and root knot nematodes in Ghana

Agroecological zone

Dry rot

(%)

RKN

(%)

Dominant component crops

Forest-transitional 81 72 Cassava, chilli/maize

Guinea-Savannah 100 24 Millet, Cassava, maize/sorghum

IDENTIFICATION AND DISTRIBUTION OF YAM NEMATODES IN GHANA

The survey of stored tubers in Ghana revealed that the prevalence of nematode was high

(>70% of sites) in both the forest and savannah agroecological zones. Root knot nematodes

were found at all sites in the Savannah, but were less prevalent in the forest zone.

Meloidogyne incognita was the species of root knot encountered and dry rot symptoms

were entirely associated with S. bradys. No other migratory endoparasite was encountered.

28

HOST RACE STATUS OF PRATYLENCHUS COFFEAE ON YAM IN GHANA

The number of P.coffeae/5g roots ranged from 0 to 743 and from 0 to 14 / 5g tubers in the

yam (Table 21). There was significant difference (P = 0.05) between the yam amd Musa

varieties. The Musa species recorded the highest number of nematodes in roots however,

Musa cv. Brodeyuo (true horn) was more susceptible. There was no correlation between

per cent root necrosis and P. coffeae population. Trials on the pest status of P. ccffeae on

yams in Ghana have been interesting.

Table 3.3 Host race status of P. coffeae on yam in Ghana

(Mean of 4-6 replicates and inoculated with 800 P. coffeae)

Yam and Musa varieties %Root

necrosis

Crack

(0-3)

Tuber

dry rot

(0-3)

No.

P.coffeae/5g

root

No.

P.coffeae/5g

tuber

D. dumetorum cv.

Nkanfo

44 0.2 0 5 0

D. cayenensis cv. Afun 43.3 0 0 35 0

D. bulbifera 62.5 0.3 0 73 0

D. alata cv. Yeremma 70 1 0 0 0

D. rotundata cv. Lili 67.5 1 0 27 10

D. esculenta 22.5 0 0 3 14

Musa cv. Asamienu 5 - - 190 -

Musa cv. Brodeyuo 17.5 - - 743 -

Table 3.4 Host race differentiation of P. coffeae from Ghana and Belize

Host species P. coffeae ex Ghana

(nematodes/g root)

P. coffeae ex Belize

(nematodes/g root)

Dioscorea togoensis 3.0 30.0

Dioscorea rotundata 1.0 341.0

Musa cv Grande Naine 256.0 6.0

P. coffeae is known to be a pest of yams in the Caribbean, Pacific and Central America but

nothing was known of its pest status on yam in Ghana, although it is widespread on

plantain. P. coffeae can multiply in roots of a number of species of Dioscorea (Table 3.3).

However, the nematode is unlikely to be a threat to yam production because tubers do not

generally support nematode reproduction. Pot studies in the UK (Table 3.4) reveal a clear

host preference between geographically distinct nematode isolates and shown that the

Ghanaian population of P. coffeae, is a Musa host race and contrasts with Central

American populations which do cause dry rot in yams. Musa varieties appear to differ in

their susceptibility to Ghanaian populations of P. coffeae; Grande Naine is the least

susceptible of the 3 varieties tested.

29

PROJECT PUBLICATIONS

Book chapter

Kwoseh, C.K., Plowright, R.A. and Bridge, J. (2001) Scutellonema bradys on Dioscorea

spp. In: Evaluating Plants for Resistance and Tolerance to Plant Parasitic Nematodes,

Starr, J.L, Cook,R. & Bridge, J. (Eds.). CAB International, Wallingford (In press)

Conference and Workshop Proceedings

Plowright, R.A. and Kwoseh, C.K. (1998) Farmers perceptions of nematode disease in

yams in Ghana and the prevalence of endoparasitic nematodes in stored stored tubers.

Proceedings the 24 International Nematology symposium, 4-9 August 1998, Dundee.

Kwoseh, C.K., Plowright, R. A., Stanfield, J. & Asiedu, R. (1998) Culturing Scutellonema

bradys on yam tuber slices. In: Proceeding of the 7th Triennial Symposium of the

International Society for Tropical Root Crops - Africa Branch, Cotonou, Benin, 11 - 17

October 1998. Poster.

Kwoseh, C.K., Plowright, R.A. and Bridge, J. (2000) Nematode pests of yams (Dioscorea

spp.) in Ghana. Proceeding of Workshop: Uptake by Farmers of Yam Research

Recommendations, Kumasi, Ghana, 16 March 2000, Natural Resources Institute,

Greenwich University, UK.

PhD Thesis

Kwoseh, C.K. (2000) Identification of resistance to major nematode pests of yams

(Dioscorea spp.) in West Africa. PhD thesis. Reading University. 200pp.

Internal Presentation

Kwoseh, C.K. 2000. The yam nematode (Scutellonema bradys) in West Africa and the

potential for its control through resistance. Presentation at DFID’s Crop Protection

Programmes’ Root Crops Cluster meeting, Natural Resources Institute, Greenwich

University, 19 September, 2000.

Reports

Meerman, J.C., Speijer, P.R., Asiedu, R. (1999) Establishment of the geographic

distribution of yam nematode pests in Nigeria, Ghana and the Republic of Benin. - in

collaboration with C. Kwoseh, J. Mudiope, R. Plowright, T.E. Sangoyomi. IITA annual

report for 1999.

Meerman, J.C., Speijer, P.R., Asiedu, R. (1999) Improvement of the methodologies for

screening of yam host plant response to nematodes. – in collaboration with R. Plowright,

C.Kwoseh. IITA annual report for 1999.

Meerman, J.C., Speijer, P.R., Asiedu, R. (1999) Screening for resistance in yam

germplasm to nematodes affecting yam - in collaboration with R. Plowright, C.Kwoseh*,

J. Mudiope. IITA annual report for 1999.

30

CONTRIBUTION OF OUTPUTS

This work has contributed greatly to the understanding of important pests of yams in West

Africa. Good sources of resistance have been identified in Dioscorea dumetorum but barriers

to interspecific hybridisation mean that this can not be exploited in the short term. Although

quantitative variation in susceptibility has been identified, it would seem unlikely that useful

nematode resistance will be found in D. rotundata itself. The development of novel cultural

control methods with farmers, based around the issues of clean seed and crop rotation is likely

to be more fruitful in the immediate future, particularly since farmers already utilise such

methods.